1. Field of the Invention
This invention provides a loudspeaker system having a primary driver and an active radiator sealed within an enclosure where the sound pressure level generated by the primary driver is tunable by adjusting the operational characteristics of the active radiator.
2. Related Art
A loudspeaker system, also known as an audio transducer, converts electrical energy into acoustical energy to generate sound. A loudspeaker system includes at least one “primary” transducer or driver that is mounted into an enclosure. The term “primary” generally indicates that the driver is connected to a signal source such as an amplifier or a crossover network. FIG. 1 shows a cutout view of a typical driver 100 illustrating some of its electromagnetic components. The driver 100 includes a magnet 102 and a voice coil 104 with two leads 106. The voice coil 104 is wound cylindrically around a tube like cylinder 108 and placed within an air gap 110. The tube like cylinder 108 is coupled to a diaphragm 112 that is supported by a suspension 114 and a spider 116. A dust cap 118 may be provided over the cylinder 108. The outer ends of the suspension 114 and the spider 116 may be coupled to a basket 120 to ensure that the voice coil 104 moves back and forth substantially along the axial direction. The two leads 106 from the voice coil 104, for example, may be connected to an audio amplifier that provides current through the voice coil 104 that is a function of the electrical signal to be transformed by the driver 100 into an audible, sub-audible or subsonic pressure variation. As the electrical signal from the amplifier passes through the voice coil 104, the interaction between the current passing through the voice coil 104 and the magnetic field produced by the permanent magnet 102 causes the voice coil 104 to oscillate in accordance with the electrical signal and, in turn, drives the diaphragm 112 and produces sound. As such, the driver 100 converts the electrical signal source into acoustical energy to produce sound.
A loudspeaker system typically has a driver housed in a ported enclosure or a sealed enclosure. The ported enclosure has an opening to allow sound waves to push in and out of the enclosure as the diaphragm of the driver oscillates back and forth. With the sealed enclosure, however, air inside the sealed enclosure compresses and expands as the diaphragm of the driver oscillates back and forth. In some instances, the sealed enclosure may be provided with a primary driver and a passive radiator. As discussed above, the primary driver has electromagnetic components to convert the electrical signal source into acoustical energy to produce sound. In contrast, the passive radiator has a diaphragm but no other electromagnetic components. This allows the diaphragm of the passive radiator to freely vibrate based on the pressure differential inside the sealed enclosure imparted by the primary driver. As the diaphragm of the passive radiator expands the net internal volume of the sealed enclosure increases to ease the pressure differential inside the sealed enclosure. The passive radiator may be incorporated in the sealed enclosure to improve the low frequency extension of the primary driver. This allows the diaphragm of the primary driver to extend further to increase the low frequency response.
With a sealed enclosure, the passive radiator and the primary driver share the same enclosure or the same acoustic-internal volume of the enclosure. The air compression and rarefaction caused by the primary driver push and pull on the diaphragm that is freely coupled to the passive radiator. Operating characteristics (excursion properties) of the passive radiator indicate how much force may be needed to push and pull on the diaphragm of the passive radiator. Many factors may define the operating characteristics of the passive radiator such as mass of the diaphragm, surface area of the diaphragm, material, etc. The operating characteristics of the passive radiator may partly determine the characteristics of the pressure changes within the enclosure and may have an effect on the overall performance of the primary driver. In other words, the passive radiator's resistance to push and pull movement may affect the overall performance of the primary driver. For example, if the passive radiator is very massive, then there may be greater resistance. If such is the case, the enclosure may be subject to a higher pressure, thereby affecting the overall performance of the primary driver.
One of the problems with a passive radiator is that its operating characteristics are fixed. In other words, once the loudspeaker system is constructed with a passive radiator, the operating characteristics of the passive radiator may not be changed without changing the mechanical properties of the passive radiator. Put differently, in the design phase of the loudspeaker system, appropriate design parameters are selected for a desired operating characteristic, such as mass, surface area, compliance of suspension, and material for the passive radiator. Once the design parameters of the passive radiator have been selected, however, they cannot be later changed.
Accordingly, there is a need for a loudspeaker system that may vary the operating characteristics of a passive radiator without altering mechanical properties of the passive radiator. This way, by varying the operating characteristics of the passive radiator, the overall output of the primary driver may be varied as well to improve the performance of the loudspeaker system.